A citizen watchdog slapped the beleaguered Port Authority with a lawsuit because the agency has refused to hand over financial information about the rebuilding of the World Trade Center.

Washington Heights resident Margaret Donovan, who runs the nonprofit Twin Towers Alliance, says she’s focused on “investigating the documents that were underlying the program at Ground Zero — particularly given the ballooning costs to the public and in particular to the region’s commuters,” in her new Manhattan civil suit.

The authority has already spent $7.7 billion on the site.

Donovan, 62, wants to know what WTC developer Silverstein Properties, which she says was awarded $4.55 billion in insurance payments, has invested in the project.

The suit comes as the PA is poised to vote this month to give Silverstein a $1.2 billion loan guarantee for construction of the 72-story 3 WTC.

The Port Authority finally denied her Freedom of Information Law request last December, 28 months after she filed it, claiming the material was exempt under agency bylaws, according to court papers.

The rejection is a message to taxpayers who subsidize the redevelopment of Ground Zero “that arrangements between the Port Authority and Silverstein Properties is none of our business,” Donovan gripes in her suit.

A rep for the PA did not immediately comment. A Silverstein spokesman said of the $4.5 billion in insurance money awarded to the developer “$3 billion went directly to the Port Authority, including$1.5 billion in rent since 9/11, and another $1.5 billion for the PA-controlled redevelopment of WTC towers 1, 5 and retail space.”